Collective behavior during the exit of a wetting liquid through a network of channels.

The exit of a wetting fluid from a thin microchannel into a sudden expansion is studied experimentally. In the case of the exit from a single channel, the advancing interface converges to a parabolic shape after an initial transient, in accordance with the lubrication limit analysis of a spreading drop. The experiments are then repeated for the exit from two parallel channels. At early times, the two exiting drops behave independently and display the same evolution as a single exiting droplet, while at late times we recover a single parabolic profile. The transition between the early and late states is due to the merging of the two drops, which is associated with a sudden increase in the flow rate. This is the signature of a collective effect which acts to redistribute the fluid spatially. Finally, the experiment is generalized to the case of seven parallel channels where a cascade of two-by-two mergings is observed, indicating that local interactions dominate the dynamics which lead to the global state of the system.